\$\begingroup\$I just now learned that Data Science.SE's top tag is machine learning, perhaps there is overlap we should look into and possibly suggest to post there in some of these scenarios.\$\endgroup\$
– PhrancisApr 15 '18 at 1:42

I'll replace the "Machine Learning" parts of your question with "code" to keep it simple. After all, that's what it is.

"My code is learning too slow"

But it works, right? If that's the only issue, it's definitely on-topic. Classic case of a performance question.

"My code is not learning at all"

The entire purpose of ML code is to learn. If it doesn't learn at all, it's broken. Broken code has no place here.

"The (training/cross-validation/test error) in my code is too big"

That's an accuracy problem and where the trouble starts. How good is good enough? Speech recognition with an accuracy of 5%, that would be broken. After all, with such an accuracy it can't be working as intended. But where do we draw the line?

If the question is phrased as "The accuracy is not good enough" it reads an awful lot like programming-challenge questions where not all test cases are completed successfully. Which means the code is broken and not ready for review.

Another grey area would be questions saying "I currently get an accuracy of 70%, which is pretty good. Can it go even higher?".

So I guess it partly depends on how it's phrased. And I don't like that one bit. This is the major question we should be focussing on, I think.

\$\begingroup\$I added your "I currently get an accuracy of 70%, which is pretty good. Can it go even higher?" to the question. I also added a link to a related topic from a few years back.\$\endgroup\$
– Simon Forsberg♦Apr 14 '18 at 15:56

\$\begingroup\$"My Machine Learning is running too slow" may often be an algorithm choice, implementation detail or problem-framing issue in ML. I don't know if those would be on-topic here (I've hopped over from Data Science)?\$\endgroup\$
– Neil SlaterApr 15 '18 at 8:26

\$\begingroup\$@NeilSlater Pointing out the flaws in an algorithm can be part of a review. We even have the algorithm tag.\$\endgroup\$
– MastApr 15 '18 at 8:56